Malaysia Airlines confirmed that it lost contact with Flight 17 and that the plane's last known position was over Ukrainian airspace, the airline said on Twitter.

The aircraft was \"shot down\" over Ukraine by \"terrorists\" operating a Buk surface-to-air missile system, according to the Facebook page of Anton Gerashchenko, adviser to the Ukrainian Interior Ministry. There were 280 passengers killed as well as 15 crew members, Gerashchenko's post reads.

\"We do not exclude that the plane was shot down and confirm that the Ukraine Armed Forces did not fire at any targets in the sky,\" Ukrainian President Petro Poroshenko said, according to his website.

The jet is a Boeing 777, according to Interfax. The plane reportedly went down near the border between Russia and Ukraine.

\"We are aware of reports on MH17. We're gathering more information,\" Boeing said on Twitter.

\"I am shocked by reports that an MH plane crashed. We are launching an immediate investigation,\" Malaysian Prime Minister Najib Razak said in another tweet.

News of the Malaysian plane comes in the same week that Ukrainian officials said a Russian fighter shot down a Ukrainian military transport plane while the aircraft was in in Ukrainian airspace.

The route the Malaysian plane was on, between Kuala Lumpur and the Netherlands, is a common one, CNN aviation safety consultant Mary Schiavo said Thursday. She said that the plane was flying over a troubled area and that close communication with air traffic controllers would be a key necessity.

In hostile or disputed areas, \"any alteration from your course, and you can have a problem,\" she said.

Russia-Ukraine dispute

Tensions have been high between Ukraine and Russia since street protests forced former pro-Moscow President Viktor Yanukovych from power in February. Russia subsequently annexed Ukraine's southeastern Crimea region, and a pro-Russian separatist rebellion has been raging in Ukraine's eastern Luhansk and Donetsk regions..

Ukrainian forces have been struggling to quell the separatist unrest. Ukraine's government has accused Russia of allowing weapons and military equipment, including tanks, to cross the border illegally into the hands of pro-Russian separatists.

The Pentagon said Wednesday that Russia now has 12,000 troops on the border with Ukraine, as well as some heavy weapons. The troop numbers had fallen to about 1,000 previously from a high of an estimated 40,000 forces earlier this year.

On Thursday, CNN reported that Ukrainian officials said a Russian fighter shot down a Ukrainian jet Wednesday as the jet flew in Ukrainian airspace.

Tensions are high over that incident, separate from the breaking news of the Malaysian flight Thursday.

Airline's troubles

On top of that, the report of a downed Malaysian flight marks the second time this year that Malaysia Airlines has faced an incident involving a plane.

On March 8, Malaysia Airlines Flight 370 disappeared. That plane had 239 people on board. Searchers have found no trace of 370 or its passengers, despite extensive search efforts.

Flight 370 probably flew into the southern Indian Ocean on autopilot with an unresponsive crew, Australian authorities said last month.

During the early phase of the search for Flight 370, aircraft and ships scoured vast stretches of the surface of the southern Indian Ocean but found no debris.

Pings initially thought to be from the missing plane's flight recorders led to a concentrated underwater search that turned up nothing.

A new underwater search, farther south, will be broadly in an area where planes and vessels had already looked for debris on the surface of the water. It is expected to begin in August.